1. Field of the Invention
The present invention relates to the field of flexible printed circuits.
2. Prior Art
Flexible printed circuits are now well known and are finding increased application in various types of products. Such circuits provide excellent flexibility and fatigue life, being ideal for applications where flexing throughout the service life of the circuit is a functional requirement, as well as applications where flexing is a requirement primarily to facilitate assembly and disassembly, to accommodate dimensional variations, etc. By way of example, flexible printed circuits formed in a rolling loop are commonly used for coupling printhead drive signals to the reciprocating printhead of dot matrix and other types of printers. Typical applications where flexibility is primarily required to facilitate assembly and disassembly include the use of the flexible printed circuit in place of a wire harness, as a flexible printed circuit provides the same function with reduced weight, greater flexibility and greater reliability, and with controlled and repeatable coupling between lines, an important consideration whenever signals which may include high frequency components are to be coupled through the circuit.
Conventional flexible printed circuit materials include as the flexible plastic film Kapton, Nomex and Teflon (trademarks of DuPont). These materials provide excellent flexibility, stability and heat resistance, and are now readily bondable to copper sheets for the formation of the printed circuit, and to themselves to provide an insulating layer for the printed circuit and/or to facilitate the construction of multilayer boards. These materials however, are very difficult to plate, and accordingly present special problems when one attempts to form interconnects between layers of the printed circuit. In particular, in conventional printed circuit board fabrication, interconnects between layers are generally formed by drilling thru-holes through pads of the layers to be interconnected, and of course all substrates aligned therewith, providing a flash of electroless plating through the thru-hole, and then electroplating through the thru-hole to provide the interconnect. The flexible printed circuit materials however, cannot be electroless plated without special preparation of the material, which of course must be done after the thru-hole is drilled, as it is the fresh material exposed by the drilling operation which must be plated. Accordingly, the formation of plated thru-holes in flexible printed circuits is an expensive, time-consuming operation, generally requiring special equipment and skills. In addition, because this technology is relatively new, it is not nearly as well developed as is the formation of plated thru-holes in conventional printed circuit boards, and therefore the results are sometimes less than desired in spite of the special processing used.
As an alternative to the use of plated thru-holes in flexible printed circuits, eyelets are sometimes used whereby the eyelets are inserted through the drilled thru-holes and flared, the eyelets making electrical contact to the exposed copper at each side of the board, the integrity of which contact is assured by soldering each side of the eyelet to the associated printed circuit. While the use of the eyelet eliminates the problem of forming plated thru-holes, insertion and flaring of the eyelets is itself a time-consuming process, and of course connection to any intermediate printed circuit layers in a multilayer flexible printed circuit can only be achieved through the use of eyelets by removing all layers thereabove at some stage of the manufacturing proces prior to insertion of the eyelet and flaring and soldering thereof. At best, eyelets also tend to be relatively large in comparison to printed circuit line widths, etc. of circuits made in flexible form, and both eyelets and plated thru-holes have the disadvantage of creating a local rigid spot in an otherwise flexible sheet, tending to concentrate stresses during flexing of the circuit at the junction between the rigid spot and the adjacent flexible circuit.
U.S. Pat. No. 4,338,149 discloses a process for making circuit boards having rigid and flexible areas. The resulting circuit is comprised of the thin flexible layers in both the flexible and rigid portions of the circuit, the rigid portions being made rigid by additional rigid layers bonded thereto. Thus any thru-holes through the rigid portions necessarily pass through the flexible layers (e.g., flexible materials bonded as part of the sandwich) and accordingly the same interconnect problem exists as heretofore discussed.
In U.S. Pat. No. 4,318,954, printed wiring board substrates for ceramic chip carriers are disclosed. The problem addressed by that invention is the differential thermal expansion between conventional printed circuit boards and ceramic chip carriers. In order to reduce the coefficient of thermal expansion in the plane of the circuit board, a graphite reinforced support member is sandwiched between board layers to constrain the expansion rate of the sandwich in the plane of the circuit board. Because the graphite reinforced support member is electrically conductive, it is necessary to insulate the support member from a plated thru-hole. This is accomplished by drilling an oversized hole in the support member and filling the perimeter of the hole with an electrically nonconductive filler, such as an epoxy washer or any other suitable insulating substances having a low coefficient of thermal expansion. Other low expansion reinforcements such as aramid fibers do not require a nonconductive filler. The nonconductive filler of course, is for electrical insulation purposes only, with the entire circuit formed by the process being rigid.